Telecommunications access systems provide for voice, data, and multimedia transport and control between the central office (CO) of the telecommunications service provider and the subscriber (customer) premises. Prior to the mid-1970s, the subscriber was provided phone lines (e.g., voice frequency (VF) pairs) directly from the Class 5 switching equipment located in the central office of the telephone company. In the late 1970s, digital loop carrier (DLC) equipment was added to the telecommunications access architecture. The DLC equipment provided an analog phone interface, voice CODEC, digital data multiplexing, transmission interface, and control and alarm remotely from the central office to cabinets located within business and residential locations for approximately 100 to 2000 phone line interfaces. This distributed access architecture greatly reduced line lengths to the subscriber and resulted in significant savings in both wire installation and maintenance. The reduced line lengths also improved communication performance on the line provided to the subscriber.
By the late 1980s, the limitations of data modem connections over voice frequency (VF) pairs were becoming obvious to both subscribers and telecommunications service providers. ISDN (Integrated Services Digital Network) was introduced to provide universal 128 kbps service in the access network. The subscriber interface is based on 64 kbps digitization of the VF pair for digital multiplexing into high speed digital transmission streams (e.g., T1/T3 lines in North America, E1/E3 lines in Europe). ISDN was a logical extension of the digital network that had evolved throughout the 1980s. The rollout of ISDN in Europe was highly successful. However, the rollout in the United States was not successful, due in part to artificially high tariff costs which greatly inhibited the acceptance of ISDN.
More recently, the explosion of the Internet and deregulation of the telecommunications industry have brought about a broadband revolution characterized by greatly increased demands for both voice and data services and greatly reduced costs due to technological innovation and intense competition in the telecommunications marketplace. To meet these demands, high speed DSL (digital subscriber line) modems and cable modems have been developed and introduced. The DLC architecture was extended to provide remote distributed deployment at the neighborhood cabinet level using DSL access multiplexer (DSLAM) equipment. The increased data rates provided to the subscriber resulted in upgrade DLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps) to high speed DS3 and OC3 interfaces. In a similar fashion, the entire telecommunications network backbone has undergone and is undergoing continuous upgrade to wideband optical transmission and switching equipment.
Similarly, wireless access systems have been developed and deployed to provide broadband access to both commercial and residential subscriber premises. Initially, the market for wireless access systems was driven by rural radiotelephony deployed solely to meet the universal service requirements imposed by government (i.e., the local telephone company is required to serve all subscribers regardless of the cost to install service). The cost of providing a wired connection to a small percentage of rural subscribers was high enough to justify the development and expense of small-capacity wireless local loop (WLL) systems.
Deregulation of the local telephone market in the United States (e.g., Telecommunications Act of 1996) and in other countries shifted the focus of fixed wireless access (FWA) systems deployment from rural access to competitive local access in more urbanized areas. In addition, the age and inaccessibility of much of the older wired telephone infrastructure makes FWA systems a cost-effective alternative to installing new, wired infrastructure. Also, it is more economically feasible to install FWA systems in developing countries where the market penetration is limited (i.e., the number and density of users who can afford to pay for services is limited to small percent of the population) and the rollout of wired infrastructure cannot be performed profitably. In either case, broad acceptance of FWA systems requires that the voice and data quality of FWA systems must meet or exceed the performance of wired infrastructure.
Wireless access systems must address a number of unique operational and technical issues including:
1) Relatively high bit error rates (BER) compared to wire line or optical systems; and
2) Transparent operation with network protocols and protocol time constraints for the following protocols:                a) ATM;        b) Class 5 switch interfaces (domestic GR-303 and international V5.2);        c) TCP/IP with quality-of-service QoS for voice over IP (VoIP) (i.e., RTP) and other H.323 media services;        d) Distribution of synchronization of network time out to the subscribers;        
3) Increased use of voice, video and/or media compression and concentration of active traffic over the air interface to conserve bandwidth;
4) Switching and routing within the access system to distribute signals from the central office to multiple remote cell sites containing multiple cell sectors and one or more frequencies of operation per sector; and
5) Remote support and debugging of the subscriber equipment, including remote software upgrade and provisioning.
Unlike physical optical or wire systems that operate at bit error rates (BER) of 10−11, wireless access systems have time varying channels that typically provide bit error rates of 10−3 to 10−6. The wireless physical (PHY) layer interface and the media access control (MAC) layer interface must provide modulation, error correction and ARQ protocol that can detect and, where required, correct or retransmit corrupted data so that the interfaces at the network and at the subscriber site operate at wire line bit error rates.
Wireless access systems should also sustain high availability for users. A necessary part of achieving high availability telecommunications systems, such as wireless access systems, is design of critical components in a redundant fashion where an “active” side handles primary functionality while a “standby” side remains idle but quickly available in the event of failure within the primary side. Being redundant, both resources are necessarily capable of performing the same system functions.
For systems which are processor controlled, the active side typically maintains status and control information for all resources which the active side controls within a private memory. Upon failure of the active side, the standby side must begin operation within a minimal amount of time. During switch-over from the active side to the standby side for primary functionality, a primary concern is that no change is apparent (transparency) to the end user. To accomplish such transparency to the end user, the standby side is generally abreast with any and all changes on the primary side (equalization) as closely as possible in real time. As a result of equalization, the standby side is able to quickly begin processing with a duplicate copy of the status and control information which the active side was utilizing at the time of failure within the active side.
Two typical techniques are employed in order to achieve equalization of active and standby memory contents: software-based and hardware-based. With software-based techniques, the processor on the active side copies certain critical information, formats the copied information into a message form and transmits the message to the standby side processor, all under software control. The control processor on the standby side receives the message and interprets the content to determine whether the message contains equalization information to be moved appropriately into the private memory of the standby side control processor, again all under software control. However, the amount of time required to format and transfer data utilizing, the need for a messaging protocol and verification of data integrity, and the need to interpret and properly store data on the standby side all increase the overhead associated with this method. Additionally, these requirements all add to the latency (delay) associated with keeping the two sides equalized.
Latency in memory equalization has two principal effects: First, the speed at which the active side processes calls is reduced since the active side is unable to process calls faster than the rate at which associated information is transferred to the standby side and properly stored. If the active side processes calls faster than the transfer rate to the standby side, the standby side falls behind in maintaining an accurate copy of the active side's status and control information (coherency), which defeats the purpose of equalization. Second, as latency in memory equalization increases, so does the probability of data loss for data essential to resuming operations in the standby side in the event of failure in the active side. Furthermore, attempts to speed transfers between the active and standby sides to reduce latency generally require dedicating more processing time to memory equalization-related tasks, which inherently reduces the amount of processing time available for non-redundancy tasks (e.g., call processing).
With hardware-based techniques for active and standby memory equalization, the active and standby components are typically very tightly coupled—in some instances to the extent that both processors are synchronized for every instruction. Since both sides operate on exactly the same instruction at exactly the same time, both side are kept equalized. However, this technique requires considerable expense to implement and, although guarding against hardware failures, suffers the side effect of vulnerability to software failures. For example, a logic flaw on the active side will be exactly duplicated on the standby side, corrupting potentially mission-critical data even as both sides are kept equalized.
There is, therefore, a need in the art for a memory equalization technique which reduces equalization latency over software-based equalization methods while avoiding the expense and software error vulnerability of hardware-based equalization methods.